Retrievable pressure sensor

ABSTRACT

A retrievable pressure sensor for the in situ measurement of pressure of a process fluid in a pipe/chamber, and an associated method. The sensor includes a pressure-transferring arrangement which includes a device for sealing attachment in an opening in a wall of the pipe/chamber, a cavity containing a pressure-transferring fluid, a first separation diaphragm for separating between the process fluid and the pressure-transferring fluid, and a second separation diaphragm defining an outer boundary of the cavity containing the pressure-transferring fluid. The sensor further includes an outer sensor part having a pressure sensing element, a second cavity containing a pressure-transferring fluid in pressure communication with the pressure sensing element, a third separation diaphragm bounding the second cavity, the third separation diaphragm being configured for pressure communication with the second separation diaphragm for transferring pressure between the pressure-transferring fluid in the cavity and the pressure-transferring fluid in the second cavity.

INTRODUCTION

The present invention relates to a retrievable pressure sensor, as wellas to methods for retrieving the sensor. In particular, the inventionrelates to a pressure-transferring arrangement for use in a retrievablesensor.

BACKGROUND

Retrievable sensors are sensors which can be replaced while processfluids are present in the area in which the pressure is to be measured.Such pressure sensors are attached in the pipe wall by way of a flangeor bore, for example. It is known to use valve mechanisms for blockingthe flow to enable the replacement of the pressure sensor. In practice,the use of valves has proven to be difficult as they are left in an openposition and subject to fouling by the process fluid. It is therefore agreat risk that the valve is not operable when it is finally needed forthe replacement of a pressure sensor, after some 10-15 years, forexample. Moreover, when valves are used it is difficult to avoiddischarge of process fluid into the environment.

It is therefore a need for a pressure sensor having a reliable structuremaking it possible to replace pressure sensors in overpressureenvironments in a simple manner without rendering the pressure sensorsinoperable and without having to open up the pipe wall or suspend theproduction/process. At the same time, the pressure sensor has to meetstrict safety requirements.

SUMMARY OF THE INVENTION

The present invention provides a solution to the above problems. Thepresent invention relates to a design enabling the replacement ofpressure sensors in overpressure scenarios, e.g. in subsea oil relatedapplications, process plants, etc.

The invention involves the use of a pressure-transferring arrangement(pressure pad) 2 as a transfer mechanism between the pressure sensorelement 4 and the process fluid 3 in which the pressure is to bemeasured. The pressure-transferring arrangement is attached in anopening in a wall of the pipe/chamber and remains in place when thesensor part is removed. The pressure-transferring arrangement 2 ishermetically welded to the pipe 1 or attached by way of a flange orotherwise in order to ensure a sealing engagement against the pipe walland process fluid inside the pipe. The pressure pad 2 has a separationdiaphragm at each end. The diaphragms may be equal or different. Eachseparation diaphragm has an associated diaphragm seat. The seat holes inboth diaphragm seats are small and few enough for the respectivediaphragms to flatten onto their respective seats and then be able tosupport the entire pressure from the process fluid 3 and an ambientpressure, respectively, when the outer sensor structure is removed. Thepressure sensor structure includes two diaphragms 8 located adjacent toeach other and serving as a transfer mechanism for the pressure of thetransmission oil 7 and in the volume 4 in which the pressure element islocated. Especially in subsea overpressure environments the requirementsof safety and reliable design are high. The present invention may thenbe provided with double barriers through the use of additionaldiaphragms in the pressure pad, for example.

In an aspect, the invention provides a retrievable pressure sensor forinstallation in an opening through a pipe or chamber wall for the insitu measurement of pressure of a process fluid in the pipe/chamber. Thepressure sensor includes a pressure-transferring arrangement/pressurepad comprising: a device for sealing attachment in an opening in a wallof the pipe/chamber, a cavity containing a pressure-transferring fluid,a first separation diaphragm for separating between the process fluidand the pressure-transferring fluid, and a second separation diaphragmdefining an outer boundary of the cavity containing thepressure-transferring fluid. The pressure sensor further includes anouter sensor part having a pressure sensing element, a second cavitycontaining a pressure-transferring fluid in pressure communication withthe pressure sensing element, and a third separation diaphragm boundingthe second cavity, with the third separation diaphragm being configuredfor pressure communication with the second separation diaphragm fortransferring pressure between the pressure-transferring fluid in thecavity and the pressure-transferring fluid in the second cavity, and athird volume surrounding the second and third separation diaphragms whenthe second and third separation diaphragms are in pressurecommunication.

A pressure in the third volume is adjustable. The third volume may beadapted for the injection of fluid. The pressure in the third volume isadjustable so that the sensor may be retrieved in a controlled and safemanner.

In an embodiment, the pressure-transferring arrangement may furtherinclude a first abutment surface for the first separation diaphragm,with the first abutment surface having an opening into the cavity. Thesecond separation diaphragm also has a, second, abutment surface, withthe second abutment surface having an opening into the cavity. An outersection of the second separation diaphragm may be attached to the secondabutment surface by way of fasteners so that the outer section abutsagainst and contacts a corresponding section of the second abutmentsurface.

In an embodiment, the pressure sensor includes a third abutment surfacefor the third separation diaphragm, with the third abutment surfacehaving an opening into the second cavity. A second outer section of thethird separation diaphragm may be attached to the third abutment surfaceby way of fasteners so that the second outer section abuts against andcontacts a corresponding outer section of the third abutment surface.The pressure sensor may include a biasing force pressing the second andthird separation diaphragms together so that pressure transfer may takeplace between the second and third separation diaphragms. In a furtherembodiment the pressure sensor may also include a fourth volume inassociation with the first cavity, with the fourth volume beingconfigured for receiving the pressure-transferring fluid of the firstcavity. The pressure sensor may be adapted for measuring differentialpressure or, alternatively, be adapted for measuring absolute pressure.

In a further aspect, the invention provides a method for retrieving aretrievable pressure sensor as defined above, the method comprising:increasing a pressure in the third volume until said pressure is higherthan the process fluid pressure, pulling out the second pressure sensorpart in a controlled manner until the second and third separationdiaphragms are no longer in pressure communication, reducing thepressure in the third volume to a pressure equal to or lower than theprocess pressure, and pulling out the outer pressure sensor part.

In a further aspect, the invention provides a method for retrieving aretrievable pressure sensor as defined above, wherein the retrievablepressure sensor further includes a fourth volume in association with thefirst cavity, and the method comprises reducing the pressure in thefirst cavity until the second and third separation diaphragms are nolonger in pressure communication, through the reception of fluid in thefourth volume from the first the cavity, and pulling out the outerpressure sensor part.

In a further aspect, the invention provides a retrievable pressuresensor for installation in an opening through a pipe or chamber wall forthe in-situ measurement of pressure of a process fluid in thepipe/chamber. The pressure sensor includes: a pressure-transferringarrangement comprising a device for sealing attachment in an opening ina wall of the pipe/chamber, a cavity containing a pressure-transferringfluid, a first separation diaphragm for separating between the processfluid and the pressure-transferring fluid, and a second separationdiaphragm defining an outer boundary of the cavity containing thepressure-transferring fluid; an outer sensor part having a pressuresensing element, a second cavity containing a pressure-transferringfluid in pressure communication with the pressure sensing element, athird separation diaphragm bounding the second cavity, the thirdseparation diaphragm being configured for pressure communication withthe second separation diaphragm for transferring pressure between thepressure-transferring fluid in the cavity and the pressure-transferringfluid in the second cavity, and an additional volume in association withthe first cavity, the additional volume being configured for receivingthe pressure-transferring fluid of the first cavity.

In one embodiment, the pressure sensor may include a first abutmentsurface for the first separation diaphragm, with the first abutmentsurface having an opening into the cavity. The pressure sensor mayfurther include a second abutment surface for the second separationdiaphragm, with the second abutment surface having an opening into thecavity. Further, an outer section of the second separation diaphragm maybe attached to the second abutment surface by way of fasteners in such amanner that the outer section abuts against and contacts a correspondingsection of the second abutment surface. The pressure sensor may furtherinclude a third abutment surface for the third separation diaphragm,with the third abutment surface having an opening into the secondcavity. A second outer section of the third separation diaphragm may beattached to the third abutment surface by way of fasteners in such amanner that the second outer section abuts against and contacts acorresponding outer section of the third abutment surface. The pressuresensor may further be provided with a biasing force pressing the secondand third separation diaphragms together so that pressure transfer maytake place between the second and third separation diaphragms. Thepressure sensor may be adapted for measuring differential pressure orabsolute pressure.

In a further aspect, the invention provides a method for retrieving aretrievable pressure sensor as set out above, the method comprisingreducing the pressure in the first cavity until the second and thirdseparation diaphragms are no longer in pressure communication, thepressure being reduced through the reception of fluid in the additionalvolume from the first cavity, and pulling out the outer pressure sensorpart.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the invention will be explained in moredetail with reference to the accompanying drawings, in which:

FIG. 1 shows a longitudinal section of a retrievable pressure sensorstructure according to an embodiment of the invention;

FIG. 2 shows a cross-section of a lower part of the sensor structure ofFIG. 1, with the lower part of the sensor structure being constituted bya pressure-transferring arrangement facing a pressure that is to bemeasured in accordance with an embodiment of the invention;

FIG. 3 shows the pressure sensor structure of FIG. 1, with FIG. 3 abeing an enlarged section of the area in which an outer part of thesensor structure is in pressure-transferring contact with the pressurepad, whereas FIG. 3 b shows a further enlarged section showing the twoseparation diaphragms providing the pressure-transferring contact, inaccordance with an embodiment of the invention;

FIG. 4 shows a longitudinal section of a retrievable pressure sensorstructure according to a further embodiment of the invention;

FIG. 5 shows a longitudinal section of a retrievable pressure sensorstructure according to a still further embodiment of the invention; and

FIG. 6 shows a longitudinal section of a retrievable pressure sensorstructure according to yet a still further embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention will now be described. Corresponding orsimilar elements are denoted with the same reference numbers throughoutthe drawings.

FIG. 1 shows a longitudinal section of a retrievable pressure sensorpositioned in a bore in a pipe wall 1 for the in-situ measurement of thepressure of a process fluid in the pipe. The pressure sensor may also bepositioned in an opening in a chamber wall for measuring pressure of aprocess fluid in a chamber. The pressure sensor element is located at adistance from the measurement area and disposed on the outside of thepipe wall. Pressure transfer is accomplished by way of separationdiaphragms and pressure-transferring fluid provided behind and betweenthe separation diaphragms and in pressure communication with the processfluid in the pipe.

The pressure sensor structure of FIG. 1 is comprised of an outerpressure sensor part in which the sensor element 4 is positioned, and alower pressure-transferring part (pressure pad) 2 positioned so as toface the process fluid to be measured. The pressure sensor part and thepressure-transferring part 2 are positioned in a closely adjoiningrelationship so that pressure transfer can be accomplished throughseparation diaphragms and pressure-transferring fluid. Thepressure-transferring part 2 is sealingly attached to the pipe wall andremains in place when the sensor part is removed. Thepressure-transferring part 2 may be hermetically welded to pipe 1 orattached by way of a flange or otherwise in order to ensure a sealingengagement against the pipe wall and process fluid inside the pipe.

The pressure-transferring part (pressure pad) 2 has a separationdiaphragm at each end. The diaphragms may be equal or different. Eachseparation diaphragm has an associated diaphragm seat. The seat holes inboth diaphragm seats are small and few enough for the respectivediaphragms to flatten onto their respective seats and then be able tosupport the entire pressure from the process fluid 3 and an ambientpressure. Typically, the holes/openings of the diaphragm seat are of thesame order of magnitude as the thickness of the separation diaphragms. Atypical thickness for the separation diaphragms is 0.05 mm and a typicaldiameter of the openings/holes is 0.25 mm. The pressure-transferringpart is exposed to the ambient pressure when the pressure sensor part isremoved.

The pressure sensor structure with the pressure-transferring partprovides a tight barrier when in place in the pipe. The attachment ofthe outer pressure sensor part and inner pressure-transferring part aswell as replacement procedures will be explained below.

FIG. 2 shows a cross-section of the lower part of the outer pressuresensor part of FIG. 1 which communicates with the fluid in the pipe inwhich the pressure is to be measured, as well as the area in which theouter pressure sensor part and the inner pressure-transferring partcontacts each other.

The inner pressure-transferring part, or pressure pad, is comprised of,as seen from the innermost component facing the process fluid to thecontact area, a diaphragm 5 with an underlying abutmentsurface/diaphragm seat 11 for the diaphragm, a hydraulic pipe/bore 7, anabutment surface 12 a for a second diaphragm 8 a, and the seconddiaphragm 8 a. Diaphragm 5 provides a barrier against the process fluid,and constitutes a so-called “process diaphragm” separating the innerpressure-transferring environment of the pressure sensor from theexternal process environment. On the inside thereof, the processdiaphragm 5 has a first concave abutment surface against which it canabut and which prevents the process diaphragm from rupturing should thepressure of the process fluid become excessive. In the first concaveabutment surface 11 there is an opening into hydraulic pipe 7. Hydraulicpipe 7 leads into a second opening in a second concave abutment surface12 a for the second diaphragm 8 a. The second separation diaphragm 8 ais exposed to the ambient pressure when the outer pressure sensor partis removed. The process diaphragm 5 and the second diaphragm 8 a at thecontact surface with the outer sensor element, together with the firstand second abutment surfaces and hydraulic pipe 7, defines a firstcavity containing a pressure-transferring fluid. Thepressure-transferring fluid may be a hydraulic liquid, such as hydraulicoil, for example.

In succession, the outer pressure sensor part is comprised of a thirddiaphragm 8 b, a third concave abutment surface/diaphragm seat 12 b forthe third diaphragm, a second hydraulic pipe/bore 10 in the thirddiaphragm, with the pipe leading into the pressure sensor assembly incavity 4. The third diaphragm 8 b at the contact surface with the innerpressure-transferring part, together with the third abutment surface 12b, pipe 7, and an outer boundary of the pressure sensor assembly, form asecond cavity containing a pressure-transferring fluid. Thepressure-transferring fluid may be a hydraulic liquid, such as hydraulicoil, for example. The second cavity is in pressure communication with apressure sensor 4. The outermost part of the pressure sensor part may beshaped as a flange section in order to also provide a seal around theouter end of the pipe wall opening.

The second 8 a and third diaphragms 8 b are attached to their respectiveabutment surfaces by fasteners in such a manner that an outer section 9a,9 b of these diaphragms abut against and contact a correspondingsection 9 a,9 b of the underlying abutment surfaces at all times.Preferably, the outer sections 9 a, 9 b of the diaphragms are annular.This prevents the outer sections of the separation diaphragm from losingthe contact with the abutment surface/diaphragm seat. The fasteners maybe in the form of screws with underlying springs. The separationdiaphragm is comprised by a thin foil, and the existence of a hydraulicpressure differential across the separation diaphragm is avoided so thatit will not rupture. The diaphragms used in the present invention couldbe of the type described in the patent application NO20093171 of theapplicant, which is hereby incorporated by reference, but could also beother types of separation diaphragms. Diaphragms 8 a and 8 b, being incontact with each other and forming the transfer surface between theinner and outer sensor parts, have complementary geometrical shapes.Process diaphragm 5, however, may be different from diaphragms 8 a and 8b at the contact surface.

A third volume 6 surrounds the second and third diaphragms at thecontact area for the inner and outer parts of the retrievable sensor. Inthe embodiment shown in FIG. 2, the third volume is annular. Thisannular volume may be closed and sealed off by an O-ring, for example,or another sealing means. The annular volume may also be open 13 so thatfluid can be injected as shown in FIG. 4. In use subsea, the fluid maybe seawater from the surroundings. When the annular volume is open, thepressure of volume 6 is adjustable. This pressure adjustment can beutilized for retrieving the sensor in a controlled and safe manner.

FIG. 5 shows an embodiment of the retrievable pressure sensor in which afourth volume 14 is provided in connection with the first cavity 7 inthe pressure pad. This fourth cavity 14 is adapted for receiving thepressure-transferring fluid in cavity 7 so as to enable adjustment ofthe pressure of the transfer fluid in cavity 7. The fourth cavity 14 maybe bounded by a pressure adjustment mechanism as shown in FIG. 5. InFIG. 5, this pressure adjustment mechanism includes a diaphragm inassociation with a cavity in which the pressure is controllable. Suchpressure adjustment may be utilized for retrieving the sensor in acontrolled and safe manner. The cavity may be isolated from thesurroundings. The annular volume 6 of FIG. 5, enclosing the second andthird diaphragms at the contact area for the inner and outer parts ofthe retrievable sensor, may be closed and sealed off by an O-ring, forexample, or another sealing means.

A further embodiment is shown in FIG. 6, comprising a fourth cavity 14adapted for receiving the pressure-transferring fluid from cavity 7 aswell as an opening 13 into the annular volume 6 for the injection ofwater from the surroundings. Hence, in this embodiment, pressureadjustment in both the annular volume 6 and cavity 7 is utilized forretrieving the sensor in a controlled and safe manner.

The pressure pad is shaped so as to be able to resist a large pressuredifference between the pressures of the process fluid 3 and third volume6. This prevents the pressure pad from rupturing when the pressuresensor (i.e. the part containing volume 4) is removed. When the pressuresensor has been put in place in the pipe wall, the pressure sensor isprovided with a biasing force pressing the two diaphragms 8 a and 8 btogether so that pressure transfer can be accomplished and the processpressure measured. This biasing force is sufficiently high to preventthe surface pressure on the outer surface 9 a from becoming zero evenwith the highest process pressure 3 possible.

Pressure sensor 4 measures the pressure of the process fluid in pipe 3when the pressure in the third volume 6 is lower than the pressure ofthe process fluid. Pressure sensor 4 measures the higher of the processfluid pressure and the pressure in the third volume 6. This may be usedfor calibration.

Methods for Replacing (Retrieving) the Pressure Sensor

The pressure sensor may be pulled out in the case where the pressure ofthe transfer oil 7 is lower than the pressure in the third cavity 6. Itis sufficient that the pressures in the third cavity 6 and first cavity7 are equal or that a very small pressure difference exists where thefirst cavity 7 has a slightly lower pressure than the third cavity 6.This pressure difference can be of the order of millibar. The order ofmagnitude of the pressure difference can be allowed to be in the 10 mbarrange, i.e. from 1-100 mbar.

At the contact area thereof, the second separation diaphragm 8 a is flator pressed down against its diaphragm seat when the outer pressuresensor assembly with the third separation diaphragm 8 b is pulled out,in order to prevent the second separation diaphragm 8 a from rupturing.FIGS. 3 a and 3 b are enlarged sections of the contact area for theseparation diaphragms 8 a and 8 b. In FIG. 3 b, the separationdiaphragms are parallel and plane, and consequently in a position thatallows the outer pressure sensor assembly to be pulled out.

If the second separation diaphragm 8 a ruptures, the process diaphragm 5will represent a barrier against the process fluid. The processdiaphragm 5 and the associated first abutment surface are shaped so thatthe diaphragm 5 may fully abut against the abutment surface andconsequently be able to resist extremely high pressures, up to 2000 bar,without rupturing.

In an embodiment, it is possible to provide the pressure-transferringarrangement with an additional double diaphragm assembly between processdiaphragm 5 and the first separation diaphragm 8 a. Such a design willprovide a double barrier system.

Replacement of the pressure sensor may be accomplished in variousalternative ways, as explained below.

1) A first alternative is to reduce the pressure of the process fluid 3until it is lower than the pressure in the third cavity 6. Thisrequires, however, that the process pressure is adjustable.

2) A second alternative method includes increasing the pressure in thethird cavity/volume 6 until it is higher than that of the process fluid3. There is only a small space between the pressure sensor and pipewall, so that the pressure in the third cavity 6 can be increased by wayof a pressure connection 13 as shown in FIGS. 4 and 5. When the pressurein the transfer oil 7 is lower than the pressure in the third cavity 6,this fluid pressure will provide a splitting of the separationdiaphragms 8 a and 8 b. When the pressure in the third cavity 6 ishigher than the pressure in the process fluid 3 and hence also in thecavity 7, the pressure sensor is pulled a little outwards in acontrolled manner. At the same time, in this intermediate position, alarge hydraulic force will act on the sensor. After the pressure sensorhas been controllably pulled a little outwards, the pressure in thethird volume 6 is released (the pressure is lowered), and the completeouter sensor part may thereafter be pulled out. The innerpressure-transferring part/pressure pad 2 will remain in place in thepipe wall and seal against the process fluid. Insertion of a newpressure sensor is accomplished by reversing the above process. In acontrolled manner, the pressure sensor is introduced almost all the wayin, the pressure in the third cavity/volume is increased, the pressuresensor is pushed in place against pressure pad 2, and the pressure inthe third cavity is reduced to a pressure which is lower than theprocess pressure.

Increasing the pressure in the third cavity 6 to a higher level than theprocess fluid 3 may also be used for calibrating the pressure sensor ina pressure range from the process pressure and upwards.

3) A third alternative method includes introducing a variable volume 14as shown in FIG. 5 in connection with the first volume 7 in the pressurepad 2. This variable volume 14 makes it possible to reduce the pressureof the transfer oil inside the first volume 7. In this alternative, thearrangement, introducing the variable volume and thereby the variablepressure in the first volume 7, maintains the pressure in the firstvolume 7 so that the sensor can be operated, and lowers the pressure involume 7 so that the outer pressure sensor assembly can be removed.Inserting a new pressure sensor can be accomplished by reversing theabove process. The pressure sensor is pushed in a controlled manner allthe way to the pressure pad and fastened. The pressure of the transferoil in the first volume 7 is then increased to the operating pressure.This embodiment may be used for calibrating the pressure sensor in thepressure range from the process pressure and downwards.

4) Any combination of the above alternatives is also possible. Thepressure testing and/or calibration device and method are applicableboth for differential pressure sensors and for absolute pressuresensors. The exemplary embodiments shown in the drawings and describedabove are shown and explained for a sensor for measuring absolutepressure. The pressure pad and pressure sensor structure, as well as themethods for retrieving the sensor according to the invention, are alsoapplicable for differential pressure sensors. For that case, the onlydifference is that at least two pressure sensor structures are arrangedin connection with each other and are connected for measuringdifferential pressure.

Calibration and Testing

In a measurement scenario the pressure sensor structure shown in FIG. 5and FIG. 6 will sense the process pressure P_(p) of the process fluid 3,provided that the pressure P_(S) (control pressure) in the fourth volume7 is higher than the process pressure.

The pressure sensor may be tested in situ in order to check that it isintact and still measures the correct pressure. The testing is performedin that the control pressure is made lower than or equal to the processpressure. When the pressure in the fourth volume 7 is lower than thepressure of the process fluid, the measurement pressure P₁ sensed by thesensor corresponds to the pressure P_(S) in the fourth volume 7. Intesting a known pressure is used as the control pressure P_(S), andconsequently the sensor is intact if the pressure measured correspondsto the applied control pressure. Calibration of the sensor may beperformed by using different known values for the control pressure,which is thereby sensed by the sensor. The measured pressure values arecompared with the applied pressures. This embodiment may be used forcalibrating the pressure sensor in a pressure range from the processpressure and downwards.

In a further embodiment, the pressure sensor structure shown in FIG. 6can be used for calibrating and testing the pressure sensor in apressure range from the process pressure and upwards. This requires thatthe second control pressure, P_(r), which may be applied across theannular volume 6 thereby splitting separation diaphragms 8 a and 8 b, isset higher than or equal to the process pressure P_(S). In thisconfiguration (FIG. 6), pressure sensor 4 will measure the pressure inthe annular volume 6 and may therefore be used for calibrating andtesting the pressure sensor in a pressure range/pressure interval fromthe process pressure and upwards. The measured pressure values arecompared with the applied annulus pressure(s) P_(r).

Such a device may advantageously be used for sensors operating on theseafloor in petroleum related applications. Such sensors are subject tostrict requirements on operational safety and solidity, and the costsassociated with sensor replacement are high. Testing and possiblycalibration of the sensor can be easily performed by running a ROV downto the installation and reducing the control pressure P_(S) to knownpressure values in a controlled manner while the pressure is sensed bythe sensor 4. The measured pressures are compared with the appliedpressures. An example of a known pressure value that may be used couldbe the pressure of the ocean depth at the sensor location. Such testingcan extend the operational lifetime of the pressure sensor. The pressuresensor is then only replaced when it actually does no longer function,as opposed to being replaced after a predetermined number of years.

The use of a closed hydraulic pipe and optionally a diaphragm inconnection with the control pressure P_(S) creates a closed pressuretesting structure that satisfies the strict zero discharge requirementregulations in subsea petroleum applications. An example of such astructure is shown in FIG. 5.

The testing and calibration according to the above embodiments can beperformed over small intervals within a large test and calibrationrange. The calibration and testing may be performed continuously overone or more pressure ranges/intervals, or at points within one or morepressure ranges/intervals. In subsea oil and gas applications, theprocess pressure, and thereby also the control pressure, is often in theorder of a few hundred bars, whereas the equipment and hence alsopressure sensors are designed for resisting up to 1000 bars. Thepressure sensor structure according to the present invention is able tosense small pressure changes of magnitude 1 mbar.

Various other modifications and variants are also contemplated withinthe scope of the invention, as defined by the accompanying patentclaims.

The invention claimed is:
 1. A retrievable pressure sensor forinstallation in an opening through a pipe or chamber wall for the insitu measurement of pressure in a process fluid in the pipe/chamber, thepressure sensor comprising: a pressure-transferring arrangementcomprising: a device for sealing attachment in an opening in a wall ofthe pipe/chamber, a cavity containing a pressure-transferring fluid, afirst separation diaphragm for separating between the process fluid andthe pressure-transferring fluid, and a second separation diaphragmdefining an outer boundary of the cavity containing thepressure-transferring fluid, and wherein the pressure sensor furthercomprises: an outer sensor part having a pressure sensing element, asecond cavity containing a pressure-transferring fluid in pressurecommunication with the pressure sensing element, a third separationdiaphragm bounding the second cavity, the third separation diaphragmbeing configured for pressure communication with the second separationdiaphragm for the transferal of pressure between thepressure-transferring fluid in the cavity and the pressure-transferringfluid in the second cavity, and a third volume surrounding respectiveouter peripheries of the second and third separation diaphragms when thesecond and third separation diaphragms are in pressure communication, apressure in the third volume being adjustable.
 2. The pressure sensor ofclaim 1, wherein the third volume is adapted for the injection of fluid.3. The pressure sensor of claim 1, further comprising a first abutmentsurface for the first separation diaphragm, the first abutment surfacehaving an opening into the cavity.
 4. The pressure sensor of claim 1,further comprising a second abutment surface for the second separationdiaphragm, the second abutment surface having an opening into thecavity.
 5. The pressure sensor of claim 4, wherein an outer section ofthe second separation diaphragm is attached to the second abutmentsurface by way of fasteners in such a manner that the outer sectionabuts against and contacts a corresponding section of the secondabutment surface.
 6. The pressure sensor of claim 1, comprising a thirdabutment surface for the third separation diaphragm, the third abutmentsurface having an opening into the second cavity.
 7. The pressure sensorof claim 6, wherein a second outer section of the third separationdiaphragm is attached to the third abutment surface by way of fastenersin such a manner that the second outer section abuts against andcontacts a corresponding outer section of the third abutment surface. 8.The pressure sensor of claim 1, wherein the pressure sensor is providedwith a biasing force pressing the second and third separation diaphragmstogether so that pressure transfer may take place between the second andthird separation diaphragms.
 9. The pressure sensor of claim 1,comprising a fourth volume in association with the first cavity, thefourth volume being configured for receiving the pressure-transferringfluid of the first cavity.
 10. The pressure sensor of claim 1, whereinthe pressure sensor is configured for measuring a differential pressure.11. The pressure sensor of claim 1, wherein the pressure sensor isconfigured for measuring absolute pressure.
 12. A retrievable pressuresensor for installation in an opening through a pipe or chamber wall forthe in situ measurement of pressure in a process fluid in thepipe/chamber, the pressure sensor comprising: a pressure-transferringarrangement comprising: a device for sealing attachment in an opening ina wall of the pipe/chamber, a first cavity containing apressure-transferring fluid, a first separation diaphragm for separatingbetween the process fluid and the pressure-transferring fluid, and asecond separation diaphragm defining an outer boundary of the firstcavity containing the pressure-transferring fluid, an outer sensor parthaving a pressure sensing element, a second cavity containing apressure-transferring fluid in pressure communication with the pressuresensing element, a third separation diaphragm bounding the secondcavity, the third separation diaphragm being configured for pressurecommunication with the second separation diaphragm for the transferal ofpressure between the pressure-transferring fluid in the cavity and thepressure-transferring fluid in the second cavity, a third volumesurrounding respective outer peripheries of the second and thirdseparation diaphragms when the second and third separation diaphragmsare in pressure communication, a pressure in the third volume beingadjustable, and an additional volume in association with the firstcavity, the additional volume being configured for receiving thepressure-transferring fluid of the first cavity.
 13. The pressure sensorof claim 12, further comprising a first abutment surface for the firstseparation diaphragm, the first abutment surface having an opening intothe cavity.
 14. The pressure sensor of claim 12, further comprising asecond abutment surface for the second separation diaphragm, the secondabutment surface having an opening into the cavity.
 15. The pressuresensor of claim 14, wherein an outer section of the second separationdiaphragm is attached to the second abutment surface by way of fastenersin such a manner that the outer section abuts against and contacts acorresponding section of the second abutment surface.
 16. The pressuresensor of claim 12, comprising a third abutment surface for the thirdseparation diaphragm, the third abutment surface having an opening intothe second cavity.
 17. The pressure sensor of claim 16, wherein a secondouter section of the third separation diaphragm is attached to the thirdabutment surface by way of fasteners in such a manner that the secondouter section abuts against and contacts a corresponding outer sectionof the third abutment surface.
 18. The retrievable pressure sensor ofclaim 12, wherein the pressure sensor is provided with a biasing forcepressing the second and third separation diaphragms together so thatpressure transfer can take place between the second and third separationdiaphragms.
 19. The pressure sensor of claim 12, wherein the pressuresensor is configured for measuring differential pressure.
 20. Thepressure sensor of claim 12, wherein the pressure sensor is configuredfor measuring absolute pressure.
 21. A method for retrieving aretrievable pressure sensor as indicated in claim 1, the methodcomprising: increasing a pressure in the third volume until it is higherthan that of the process fluid, pulling out the outer pressure sensorpart in a controlled manner until the second and third separationdiaphragms are no longer in pressure communication, reducing thepressure in the third volume to a pressure equal to or lower than theprocess pressure, and pulling out the outer pressure sensor part.
 22. Amethod for retrieving a retrievable pressure sensor as indicated inclaim 1, wherein the retrievable pressure sensor further includes afourth volume in association with the first cavity, the methodcomprising: reducing the pressure in the first cavity until the secondand third separation diaphragms are no longer in pressure communication,with the pressure being reduced through the reception of fluid in thefourth volume from the first cavity, and pulling out the outer pressuresensor part.
 23. A method for retrieving a retrievable pressure sensoras indicated in claim 12, the method comprising: reducing the pressurein the first cavity until the second and third separation diaphragms areno longer in pressure communication, with the pressure being reducedthrough the reception of fluid in the additional volume from the firstcavity, and pulling out the outer pressure sensor part.
 24. The pressuresensor of claim 12, wherein the third volume is adapted for theinjection of fluid.